1. Field of the Invention
The invention relates generally to the field of well log interpretation. More specifically, the invention relates to methods for determining hydrocarbon saturation in the pore spaces of subsurface rock formations at lateral distances (“depths of investigation”) greater than that of measurements made by well logging instruments that probe near-wellbore regions such as—but not limited—to nuclear magnetic resonance.
2. Background Art
Nuclear magnetic resonance (“NMR”) well logging instruments known in the art make measurements related to certain nuclear magnetization properties of subsurface fluids in the rock formations. Such properties include longitudinal and transverse relaxation times of the various fluids in the formations, and diffusion constants of such fluids. The measurements may be used for determining the fractional volume of pore space in porous formations that is filled with water (called “water saturation” and represented by Sw). Such water saturation determinations may be made independently of other water saturation related parameters, e.g., the electrical resistivity of the formations using. NMR water saturation determinations may be made, for example, diffusion-relaxation maps. As is known in the art, the fractional volume of pore space in a subsurface rock formation that is not occupied by connate water or the liquid phase of drilling fluid is generally assumed to be filled with connate hydrocarbons, including oil, gas and mixtures thereof. Such hydrocarbon content is used for, among other purposes, evaluating the probable fluid production properties of subsurface rock formations.
NMR measurements are particularly useful in subsurface formations that have large uncertainties about parameters used in determining Sw from electrical resistivity measurements (so-called “Archie” determination) such as unknown connate water resistivity (resistivity of naturally occurring water disposed in the formation pore spaces), complex pore geometry, unknown cementation factor m, unknown wettability (n), high clay content (particularly where there is high clay electrical conductivity relative to water electrical conductivity), among other factors. However, NMR well logging instruments known in the art typically measure properties of the subsurface rock formations at relatively shallow depths of investigation into the formations. Depth of investigation is used to mean the lateral distance into the rock formation from the wall of the wellbore in which the NMR instrument is disposed for measurement. At shallow depths of investigation (close to the wellbore wall), the water saturation may not be representative of the natural (undisturbed) water saturation in the formations because of infiltration of liquid phase of drilling fluid into the pore spaces proximate the wellbore. The foregoing is particularly problematic when “oil based” drilling fluids (those in which hydrocarbon is the continuous liquid phase) are used to drill through subsurface rock formations. When such oil based drilling fluids are used, the liquid phase of the drilling fluid that infiltrates the formation pore spaces is primarily oil or other liquid hydrocarbon, and such hydrocarbon may displace connate hydrocarbons, connate water, or mixtures thereof.
There is a need for techniques to use NMR well logging measurements to better estimate natural or undisturbed water saturation in subsurface rock formations.